The dangers inherent in unreinforced or improperly reinforced underground passageways, mines and the like are well documented. Various anchor assemblies have been used to reinforce underground passageways, mines and the like to minimize the hazards associated therewith. Well known anchor assemblies include full column resin systems, mechanical expansion systems and combined resin-mechanical expansion systems. The present invention pertains to those anchor assemblies using both a resin bonding element and a mechanical expansion unit. Accordingly, the following discussion will be directed to anchor assemblies of this type.
Generally, these systems include a two compartment cartridge consisting of a resin and a catalyst and an anchor assembly threaded on a bolt. An example of such an assembly is disclosed in U.S. Pat. No. 4,664,561 assigned to the assignee of the present invention. The anchor assembly therein includes a bearing plate resting on the head of the bolt and a mechanical expansion unit threaded onto the opposite end thereof. Conventional, mechanical expansion units include a camming plug and an expandable shell. Further, a bail or threaded support nut is used in conjunction with the mechanical expansion unit to support the expandable shell during installation of the bolt. The two compartment cartridge is inserted into a bore hole formed in the substrate followed by the anchor assembly threaded on a bolt. Once the resin cartridge is punctured by the insertion of the anchor assembly, the resin and the catalyst flow downwardly, mix and surround the same.
The mechanical expansion unit is activated by rotating the bolt to draw the camming plug downwardly for thereby forcing the fingers of the expandable shell into engagement with the wall of the substrate. In this manner, the bolt is rotatably secured to the substrate. The bolt is further supported in the bore hole of the substrate by the resin when hardened.
After the shell has been expanded, further rotation of the bolt causes the camming plug to be drawn down into the expansion shell and thereby create tension in the bolt shank. This forces the bearing plate against the roof of the substrate and to compress the competent strata into a beam. This procedure significantly improves the load bearing characteristics of the roof of the substrate. The tensile force on the bolt must be maintained to ensure that the strata is adequately compressed. It is desirable to determine the tensile force on the bolt at various time intervals after installations, although, conventional combined resin-mechanical anchor assemblies have been unable to do this on a cost effective basis.
It is well known that a functional relationship exists between the torque and tensile forces on a bolt. Thus, the tensile force on the bolt can be readily determined from the torque. In previously known combined resin-mechanical anchor assemblies, the resin surrounds the bolt shank as well as the threaded end portion and becomes embedded in the grooves of the threads. Once the resin hardens, it restricts the rotational movement of the bolt. Therefore, values for the torque on the bolt obtained subsequent to the hardening of the resin are not possible. There is a direct torsional relationship between the bolt, resin and bore hole wall. Conventional torque measurements produce "spring back" or twisting of that portion of the bolt shank not in contact with the resin bond.